A subscriber loop connects a telephone company central office to a demarcation point on customer premises. The subscriber loop is typically a pair of wires such as a twisted pair. Traditionally, a subscriber loop is used to carry plain old telephone service (POTS) signals between central office equipment and customer equipment.
Bridged taps are sometimes introduced into the subscriber loop. Bridged taps are typically wire splices on a cable pair. Bridged taps simplify outside plant design and administration by making a wire pair physically available for termination at different customer locations. Although only one customer typically uses the cable pair at any time, the bridged tap provides flexibility as to which particular customer is assigned to any particular cable pair. This flexibility simplifies network design that results from frequent customer moves, requests for second lines, and new constructions. Often, a bridged tap is left unterminated once installed in the subscriber loop.
Recently, services based on digital subscriber lines (DSLs) have been provided to customers over subscriber loops. DSL signals operate at a higher bandwidth than POTS signals, permitting increased access to voice, data, and video services. Also, DSL signals may be modulated to a frequency range outside the range covered by POTS signals.
Bridged taps in subscriber loops may severely attenuate DSL signals. This is because the unterminated bridged tap appears as a low impedance element over certain frequency ranges depending, in part, on the geometry of the bridged tap. If DSL signals are operating within these frequency ranges, the signals may be severely attenuated. Previous efforts to terminate bridged taps reduce the effects of notches or dips in the spectrum due to the bridged tap but result in an overall attenuation of the DSL spectrum.
What is needed is proper termination of bridged taps to allow adequate transmission of DSL signals without significant signal loss. The termination should cancel the effects of the bridged tap. Bridged tap cancellation should be adaptable to different tap geometries and subscriber loop variations as well as be economical to produce.